These data were used to examine the effectiveness of a non-lethal tool (Bioelectrical Impedance Analysis, "BIA") to estimate the physiological condition of endangered and threatened fishes in the Colorado River Basin. We conducted laboratory trials using hatchery-raised Humpback Chub and Bonytail and wild-captured Roundtail Chub, where fish were subjected to different feeding trials to elucidate a response in physiological condition and different temperature treatments to approximate field conditions. At the end of each 6-week trial fish were removed from tanks, lateral and dorsal measurements of BIA were taken, and fish were sacrificed for proximate composition analysis (lipid, protein, water, ash, dry mass, energy...